This invention relates to laser systems, particularly but not exclusively for providing output profiles suitable for laser crystallisation of semiconductor films, such as crystallisation of amorphous silicon to form polycrystalline silicon in thin film devices. The invention also relates to apparatus and methods for crystallising a semiconductor film using such a laser system.
It is well known that the use of laser crystallisation processes can enable the manufacture of low-temperature poly-silicon devices, for example for displays or other circuits. However, the laser crystallisation process can give rise to poor yields, resulting from poor uniformity of the laser crystallisation process.
One factor contributing to this poor uniformity is the use of a simple so-called top-hat beam profile. This profile has a substantially constant intensity over the width of the profile. Thus, when an amorphous silicon film is crystallised by scanning this beam across it, the amorphous silicon film is exposed to a substantially constant intensity. This intensity must fall within a very narrow range for the crystallisation process to be successful. This is because it is required that nearly all of the film is melted during the crystallisation process, and there should be no full melt-through of the amorphous silicon layer. During cooling, a portion of the layer which has experienced full melt-through will recrystallise into a fine-grained structure giving rise to reduced quality in the final product. The intensity may exceed the maximum value and thereby achieve full melt-through as a result of jitter in the output from the laser source.
Because the laser output is scanned across the sample being treated, an individual laser pulse which exceeds the maximum desired intensity will be the first laser shot applied to some areas of the substrate, but will be the last laser shot applied to other areas. The region of the substrate for which that pulse was the last shot will maintain this fine-grained structure since no further laser heating is carried out.
According to the invention, there is provided a laser system for producing a two dimensional laser output comprising: a laser source producing a two dimensional output having a first intensity profile across the two dimensional area; a lens system for modifying the intensity profile of an incident laser input and comprising a plurality of lens elements arranged at a predetermined lens pitch, the system being characterised in that an optical filter is provided between the laser source and the lens system, the filter comprising transmissive portions and opaque portions, the transmissive portions defining a repeating pattern with a pitch corresponding to the lens pitch, the optical filter modifying the input to the lens system such that the output of the lens system gives rise to a desired intensity profile at the output of the laser system.
The optical filter in the laser system of the invention enables the output from the laser source to be modified so that the output of the laser system, even with a conventional lens system for producing a top-hat profile, has a modified output for subsequent laser crystallisation.
The modification to the output of the laser system may be selected to alter the intensity profile of the output across its width. For example, the lens system may be designed to modify the first intensity profile to produce a top-hat intensity profile having a substantially constant intensity over a substantial part of the width of the profile, and the optical filter is arranged such that the output of the lens system has a tapered intensity profile over part of the width of the profile.
Alternatively, the modification to the output of the laser system may be selected to vary the shape of the output beam. For example, the lens system may be designed to reduce the width and increase the length of the first intensity profile to produce a line beam, and the optical filter may then be arranged to enable a discontinuous line beam output.
The optical filter may comprise a plate having an opening or openings, or it may comprise a patterned dielectric layer or layers provided over a substrate.
Preferably, the laser source comprises a pulsed laser source, having a semi-Gaussian profile.
The invention also provides laser crystallisation apparatus comprising a laser system of the invention and means for scanning the width of the desired intensity profile across the surface of the sample for laser treatment. The scanning may be achieved by providing the sample on a movable support.